1. Field of the Invention
This invention is concerned with refining lubricating oils. In particular, it is concerned with an improved process for solvent extraction of hydrocarbon oils to remove unstable and low viscosity index (V.I.) components therefrom. It is further concerned with the refining of lubricating oils by extraction with phenol or furfural solvent.
2. Background of the Invention
Solvent refining is used in the petroleum industry on a broad scale for the removal of low viscosity index and chemically unstable materials from petroleum lube fractions which do not contain asphalt. Solvents such as furfural, phenol, chlorex, and nitrobenzene have been generally recognized as commercially practical for single-solvent refining. All of these solvents are organic chemicals heavier than the lube fractions to be treated and capable of removing undesirable constituents by the physical action of preferential solution. In general, the choice of a particular solvent process is an economic consideration since all of them are capable of producing essentially the same finished oil within the broad range of theoretically possible operating conditions.
In a furfural extraction unit, which is typical, oil is introduced into the side of a tower containing suitable means, such as a packing, to insure intimate mixing with the solvent which is introduced near the top. The solvent being heavier flows to the bottom of the tower while the oil flows to the top. As these two flow countercurrently through the tower the solvent extracts the undesirable material from the oil. The material from the top of the tower is paraffinic and is known as the raffinate. It is the refined oil with a relatively small amount of dissolved solvent. The material from the bottom of the tower is known as the extract. It is the balance of the solvent containing the undesirable material. Both streams are transferred through a heater to separate solvent recovery towers. In these towers, which are similar to distillation columns, the solvent is vaporized and removed from the raffinate and extract. The physical nature of the process is demonstrated by the fact that it would be possible to recombine the raffinate and extract to secure a material identical in all respects to that charged to the process. This is also true of other solvent extraction processes. The refiner maintains control over the quality of the product by varying the quantity of the solvent and the temperature gradient within the extraction tower.
In spite of its industrial acceptance, there are problems associated with the solvent extraction process. For example, the tower packing becomes fouled with use, gradually reducing the efficiency of the operation and finally requiring shutdown for cleaning. Fouling is particularly severe with furfural as the solvent due to its propensity to oxidize and resinify, but phenol also tends to oxidize and in time forms deposits of oxalic acid. In addition, solvent extraction is a fairly expensive operation as it is now practiced, and there is a need for improvement of efficiency.
A publication titled "Gas Agitated Liquid Extraction Columns" by Ronald Priestley and Stephen R. M. Ellis appeared on pages 757-760 of the October 7, 1978 issue of Chemistry and Industry. Attention is called to this publication and the references contained therein for possible relevance to the present invention.
It is an object of this invention to provide a solvent-extraction process of improved efficiency for refining an asphalt-free hydrocarbon oil. It is a further object of this invention to provide means for modifying existing plants to increase throughput without altering product quality. It is a further object of this invention to provide a solvent-extraction process which conserves energy. These and other objects will become apparent to one skilled in the art on reading this entire specification including the claims thereof.